The present invention relates to a servo-controlled table positioning apparatus and more particularly to a positioning apparatus for accurately indicating when a table is at a predetermined reference location.
Servo-controlled table positioning systems are frequently employed when large-scale integrated circuits are fabricated under computer control. In one such system, a highly accurate electron beam exposure system exposes a semiconductor wafer or other work piece located on a servo-controlled table.
In order to fabricate integrated circuits, the table supporting the work piece is positioned relative to the incident electron beam. The electron beam is scanned over the work piece to expose an electron beam resist on the surface of the work piece. Each area to be scanned by the electron beam has a location determined by the table position. For accurate positioning of the work piece, the table supporting the work piece must be accurately positioned. Positioning with an accuracy of 1.0 micron or better is desirable. Such an accuracy is desirable for establishing an initial reference location of the table. The location of the table is then measured with respect to this initial reference location.
In semiconductor processing, it is often necessary to remove the work piece from the table for an intermediate step and then replace the work piece on the table for additional processing. The capability of accurately positioning the table at an initial reference location is desirable in order to insure accuracy from step to step.
Servo-controlled table positioning systems capable of such accuracy are not readily available and hence electronic compensation of the electron beam together with external measuring units such as interferometers have been employed to overcome positional errors of the table. Such systems have employed microswitches or mechanical stops for establishing the initial reference location from which coordinates are measured. However, a problem occurs in the use of microswitches or mechanical stops, because they are generally not capable of achieving the 1.0 micron or better positioning accuracy desired.
In view of the above background, it is an object of the present invention to provide a table positioning apparatus which provides accurate and repeatable initial positioning of a table at a predetermined reference location.